The overall objective is to meet a critical need: determining immediately after an incident whether individuals have received a radiation dose that could result in acute clinical symptoms. If the exposure conditions indicate that acute effects are possible, we need to be able to place individuals into appropriate dose categories rapidly and accurately, to guide therapeutic approaches and counseling. With the potential development of procedures to mitigate damage from ionizing radiation, the need to determine who is at risk is even more urgent, since such interventions are likely to most effective when used as early as possible. When exposures are found to be in a range that is unlikely to lead to acute effects, the availability of the measurement will provide reassurance and reduce the number of people entering the medical care system. There are substantial data available that indicate that in vivo EPR dosimetry, based on radiation-induced effects in teeth, has the sensitivity and accuracy to meet a major part of the needs for such after-the-fact quantitative dose assessment, providing effective screening information and immediate readouts in less than five minutes. In this project, we propose a set of related specific aims to (a) address the immediate needs and (b) pursue longer-term developments to enhance capabilities. The most urgent need is a field deployable instrument for rapid "after-the-fact" dosimetry. This will be our first priority. We initially will make the existing facility at Dartmouth fully operational for emergency dosimetry, and will use these and further improvements as the basis for the rapid development of a transportable dosimeter suitable for field use. This will be tested at Rochester, where it also will be used for extending calibrations/validations, using the expertise and patient populations there. There also are a number of feasible ways to improve the sensitivity and field deployability of the method; these will be pursued as efficiently as possible without affecting the first priorities. These include the potential development of a hand-held version of the dosimeter, and several promising methods to enhance sensitivity. Such increases in sensitivity will further reduce the time needed to make the measurements and enable us to extend the method to a lower threshold, potentially to doses well below those that can cause acute clinical effects. There also are some plausible means to complement in vivo EPR dosimetry by making measurements in vitro, using hair, finger/toenails, or very small chips of teeth. These measurements could broaden the capacity for screening, and also aid in resolution of asymmetric exposures.